The present invention relates to a method for the physical designation of semiconductor wafers according to predicted oxygen precipitation behavior for a given set of thermal process steps. This method utilizes non-destructive tests on the wafer and thus can be performed on every wafer prior to semiconductor device fabrication therein.
In the mass-production of large scale integrated circuit devices, it is highly desirable to detect material deficiencies that may produce either unsatisfactory LSI devices, or low yields of satisfactory devices, prior to the initiation of any fabrication process steps. Such process steps add cost to the individual wafers, and should be minimized for any wafers ultimately destined for scrap.
It is now well known that the existence of oxygen in single crystal semiconductor wafers such as, for example, silicon wafers, has a profound effect on ultimate device yields and device characteristics in large scale integrated (LSI) electronic circuits, especially field effect and bipolar transistor digital circuits. The presence of this oxygen in the semiconductor wafers has a beneficial effect in terms of wafer hardening (solution hardening) and impurity gettering. The wafer hardening arises due to oxygen atoms located interstitially in the semiconductor lattice acting to prevent, to some degree, the semiconductor atoms from moving within their lattice. Accordingly, these oxygen atoms sitting interstitially in the lattice act to harden the lattice and prevent wafer warpage and lattice shear. The impurity gettering arises due to oxygen precipitates formed in the lattice. Essentially, these oxygen precipitates microscopically disrupt the semiconductor lattice and form a dense localized network of lattice dislocations. Metallic impurities arising in crystal growth, wafer fabrication, wafer cleaning, and/or during subsequent hot processing steps such as epitaxial deposition (800.degree.-1200.degree. C.), dopant drive-in (900.degree.-1200.degree. C.), or chemical vapor deposition (700.degree.-1100.degree. C.), tend to migrate to these oxygen precipitate sites where they are trapped. These oxygen precipitates are typically formed by oxygen atoms leaving the interstitial solution in the semiconductor lattice to form microscopic SiO.sub.2 particles or Si.sub.x O.sub.y complexes. They are essential to removing the metallic impurities which are added in above-listed later device processing steps and which would destroy device operation.
The major phenomena affecting oxygen precipitation behavior for a semiconductor wafer of, for example, silicon, undergoing a given defined sequence of thermal steps, are the presence of undesired contaminants, the initial oxygen concentration in the lattice solution, and the grown-in or annealed microstructure of the lattice. Accordingly, during semiconductor device manufacturing via the growing of a monocrystalline boule of suitably doped silicon, much effort has been expended in the past to limit the contaminants in the as-grown boule. Likewise, much effort has been expended to control the growth process to obtain a uniform radial and axial distribution of oxygen compatible with subsequent process and device needs. However, despite improvements in crystal growing technique, there still exist variations in individual wafers sliced from a given boule that may render any given one of the wafers unsuitable for subsequent device processing. Likewise, there are clear differences between individual boules. Such differences manifest themselves due to differences in thermal processing temperatures, heating and cooling rates, furnace configurations, spacing of wafers in boats, non-oxidizing or oxidizing ambients, the intrusion of trace contaminants, etc. These differences in process details can produce profound differences in the resulting wafers.
Accordingly, it is highly desirable to be able to test each wafer cut from a boule, before or after polish, to insure that it is suitable for the fabrication of a device thereon, and prior to performing any further device fabrication steps. This testing could then be used for either marking or sorting the wafers in order to minimize the number of cost-adding process steps performed prior to scrapping the wafers. However, at the present time there is no satisfactory method for predicting the oxygen precipitation content that will exist after the performance of a thermal process step. Current manufacturing techniques generally require the dedication for destructive test purposes of a certain number of wafers from a test lot. This wafer dedication not only precludes the use of these same wafers for device fabrication, but also places undue reliance on statistical sampling analysis. Moreover, it has been found that the use of sister wafer testing for crystals is, in many cases, inaccurate due to micro-fluctuations and along across the crystal in the various atomic concentrations and lattice structure.
A variety of studies have been performed in the art directed at oxygen precipitation content in semiconductor wafers. For example, see the study by Huber et al., Satellite Symposium, Electro Chemical Society, ESSDERC 82, Aggregation Phenomena of Point Defects In Silicon, Munich, Sept. 17, 1982, wherein oxygen precipitation engineering in silicon is discussed. However, such studies do not adequately disclose methods of accurately predicting oxygen precipitation content of a wafer after a thermal process step.
The invention as claimed is intended to remedy the above-defined drawbacks. The invention solves the problem of how to predict oxygen precipitation content in a semiconductor wafer which exists after a given set of device processing thermal cycles have been performed. This invention further solves the problem of predicting oxygen precipitation content without damaging the wafer under test.
The advantages offered by the invention are that oxygen precipitation content in a semiconductor wafer can be predicted prior to the performance of thermal processing steps, thereby preventing the addition of cost-adding process steps to wafers which are to be scrapped. A further advantage to the present invention is that the oxygen precipitation content of the wafer is predicted by means of non-destructive tests performed on the wafer, thereby permitting the testing of every wafer. Thus, it is no longer necessary to dedicate certain wafers from a lot for test purposes, or to rely on statistical sampling analysis for wafer batches. A still further advantage of the present invention is that through the use of this oxygen precipitation prediction method, it is now possible to select wafers in accordance with predicted oxygen precipitation values for a given product/process application, to provide rapid feedback for the optimization of the device process and/or fabrication conditions (including crystal growth and subsequent thermal treatments), and to characterize wafers on an arbitrary standardized "figure of merit" oxygen precipitation scale, to facilitate sorting. These options can be exercised independently of a particular product process.